1. Field
The embodiments described herein are related to Radio Frequency Identification (RFID), and more particularly to managing inventory for items stored in cabinets.
2. Background
Inventory control and asset tracking of items within a container (such as cabinet or shelf) are currently managed by various modes such as barcode, item count, honor system, and/or a check in/out sheet. The problem with these systems is that they require human intervention, which is inherently flawed and prone to errors. Generally a perpetual audit is implemented to correct the errors however this is resource intensive and does not identify the root cause of the problem.
For example, prescription medications in a hospital are often stored in cabinets that can be wheeled from patient room to patient room. Accurate inventory control of the medications is important to ensure that the medications are not stolen and to be sure that they are restocked when needed. The flaws with conventional inventory control processes can lead to significant consequences if inventory is removed without permission or not restocked. Moreover, it is outside the ability of conventional techniques to ensure that the correct medication in the correct amount is provided to each patient.
As a result, current solutions, e.g., barcodes, have been replaced by RFID solutions. An RFID solution can comprise RFID stickers or labels, i.e., a sticker or label that includes and RFID tag, affixed to the inventory items, e.g., bottles. Information related to each item can then be stored in the tag and read by a scanner. For example, the tag's unique identification number can be associated to a central database and, e.g., used in tracking certain items or for other purposes. In order to read the tags, a number of antennas are placed within the cabinet. The antennas are interfaced with the scanner, which can be in, or on the cabinet. The scanner sends interrogation signals via the antennas to the tags requesting the information stored thereon. The tags respond with a signal that is also picked up by the antennas and forwarded to the scanner.
It will be understood that the tags can be active or passive tags. Active tags have a battery on board; however, conventional active tags are bulky, in part due to the battery, and therefore are not optimal for many cabinet applications. Passive tags on the other hand do not include a battery and can therefore be made quite small and can therefore be preferable for cabinet applications. Passive tags are powered via the interrogation signals received form the scanner.
In some instances, different antennas can be used to transmit interrogation signals and to receive the tag replies. In general, however, conventional RFID solution employ a combined transmit and receive antenna system for simplicity, reduction of antennas and to follow the traditional concept that the most effective receive antenna is the one that is capable of illuminating the tag. In any event, the antennas must be placed so as to increase the likelihood that the interrogation signals can be received by all tags, and to ensure that all of the responses can be received and deciphered.
Conventional cabinet solutions employ a conductive chamber design to contain the RF energy associated with the interrogation signals within the chamber for increased field strength and spatial diversity; however, many such conventional designs can suffer from poor results obtained due to the static nature of the interrogations. In an application where the field is static, a tag may lie in a RF null created by multipath, resulting in a failed interrogation. Since most cabinet solutions are designed for asset tracking or secure inventory control, a form of a lock is used to secure the contents during the RFID interrogation and when not in use to prevent fraudulent activity. Since access to the cabinet's contents is prohibited during a RFID interrogation, the cabinet's doors and/or drawers need to be locked resulting in a static read of the cabinet's contents. Accordingly, conventional cabinet applications by design are static during the RFID interrogation process and suffer from occasional failed interrogation due to a tag being located within a null.
Further, many conventional solutions use the traditional combined transmit/receive antenna configuration. This configuration works well in traditional applications where the scanner antenna radiates into open space and objects are in the far-field region for minimum scanner antenna detuning. Far-field is described as a boundary region where the angular field distribution is essentially independent of distance from the source; however, in applications where the tag is in the near-field, such as in cabinet applications, the traditional combined transmit/receive antenna approach and combined transmit and receive systems suffer greatly from the scanner's inability to listen to the tag's response.
As tagged product enters the scanner's near-field region, it has an adverse effect on the scanner's antenna tuning resulting in reduced scanner receiver sensitivity. This results in scanner antenna detuning and presents a challenge for the scanner's receiver in terms of energy reflected back into the scanner receiver competing with energy reflected back by the tagged items.
Further, as will be understood, typical RFID systems require the scanner to receive a backscatter signal from the tag while transmitting. Simultaneous transmission and reception causes high levels of RF energy to enter the receiver, ultimately limiting the receiver sensitivity. Existing system designs attempt to solve this problem by either minimizing the signal reflections back into the receiver or by using separate transmit and receive antennas. Minimizing signal reflections via component selection has practical limitations. Using separate antennas increases the system cost and requires additional space.
Still further, RF signal propagation in contained environments is not well defined, with huge amplitude variations in resonant versus null locations within a drawer or chamber. When RFID tags are placed in a chamber's null locations, the tags cannot be powered and cannot be read/interrogated, ultimately causing the overall application to fail.
Another problem exist when a tag is in its minimum field strength (such as between two transmitting antennas) with respect to its ability to turn on and participate in the interrogation. When this occurs the scanner may be unable to detect the tags faint responses resulting in a failed interrogation. This is a common problem in a high product/tag density application where high concentration of items exists within the RF Tx and Rx paths.
Another problem with conventional solutions occurs when the items being tracked include large amount of liquids. Conventional RFID cabinet systems typically use the electric field to communicate to beam powered RFID tags. Depending on frequency used, some frequencies can be greatly attenuated by liquid items within the cabinet resulting in failed interrogation due to insufficient field strength.
Still another problem is that the tags have an effective area that is much larger then the real area and is normally at least ¼ wavelength of the frequency. RFID application in particular are very sensitive to this due to the fact that the RFID tags are typically place on various items that can greatly reduce the tags efficiency due to intrusion of its effective area. This problem is compounded in applications that do not adhere to any item discipline since the item itself can come into contact with the RFID tag.
These and other problems/issues can significantly reduce the effectiveness of inventory tracking using RFID enabled cabinets.